The present disclosure relates to thermoelectric generators (TEGs), and particularly TEGs for use in motor vehicles.
Fuel efficiency of motor vehicles can be improved by integrating a thermoelectric generator (TEG) in the exhaust system. The TEG converts waste heat from the internal combustion engine into electricity using the Seebeck Effect. The typical TEG includes a hot-side heat exchanger, a cold-side heat exchanger, thermoelectric materials and a compression assembly. The heat exchangers are typically in the form of metal plates with high thermal conductivity. The temperature difference between the two surfaces of the thermoelectric module(s) generates electricity using the Seebeck Effect. When hot exhaust from the engine passes through the TEG, the charge carriers of the semiconductors within the generator diffuse from the hot-side heat exchanger to the cold-side exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential Va (FIG. 2) while the heat transfer drives a current. With exhaust temperatures of 700° C. (˜1300° F.) or more, the temperature difference between exhaust gas on the hot side and coolant on the cold side is several hundred degrees. In certain automotive TEGs, this temperature difference is capable of generating 500-750 W of electricity.
The compression assembly aims to decrease the thermal contact resistance between the thermoelectric module and the heat exchanger surfaces. In coolant-based automotive TEGs, the cold side heat exchanger uses engine coolant as the cooling fluid, while in exhaust-based TEGs, the cold-side heat exchanger uses ambient air as the cooling fluid.
By way of example vehicle exhaust system 10, shown in the generalized schematic of FIG. 1, includes an exhaust pipe 12 that directs heated exhaust gases from an internal combustion engine 14 to an exhaust system outlet 16, such as a tailpipe for example. The exhaust system 10 may include additional exhaust components positioned between the engine 14 and the outlet 16, such as mufflers, resonators, catalysts, etc. A TEG 20 is incorporated into the exhaust system and in particular integrated into the exhaust pipe 12 to transform heat generated by exhaust gases into electrical energy/power. The TEG 20 can store this generated electrical energy in a storage device S, such as a rechargeable battery and/or may provide the electrical energy to various vehicle systems VS1-VSn as needed. The vehicle systems VS1-VSn may include engine controls, exhaust system controls, a door lock system, window lifting mechanism, interior lighting, etc. A controller 22 may be provided to control the storage and usage of the electrical energy generated by the TEG 20.
In one example, the thermoelectric unit 20 is constructed from at least one of semi-conductor and semi-metal materials that have specific upper and lower temperature limits of efficient operation. Exposure to excessively high exhaust gas temperatures over this upper limit can damage these materials, and exhaust gas temperatures that are below the lower limit can result in ineffective electrical power generation. Consequently, in prior systems it has been necessary to include a temperature control device 30 positioned in the exhaust pipe 12 upstream of the TEG 20. The temperature control device 30 can be in the form of a cooling device 30a that cools heated exhaust gases to temperatures within a specified temperature range that is between the upper and lower temperature limits of materials used to construct the TEG 20. These cooled exhaust gases are then communicated to an inlet 32 to the thermoelectric unit 20. The exhaust gases pass through the TEG 20, waste heat from the exhaust gases is transformed into electrical energy, and then the gases exit the thermoelectric unit 20 via an outlet 34. This configuration comprises a non-bypass arrangement in which all of the exhaust gases flow through the thermoelectric unit 20.
The cooling device 30a can include many different types of cooling components. For example, the cooling device 30a could be a fluid cooled heat exchanger, or could include air or water injection for cooling. Optionally, the cooling device 30a could comprise an air gap pipe combined with air injection or forced air cooling, which can provide both cooling and a potential reduction in thermal inertia to avoid faster heat up. The cooling device 30a may be configured to incorporate the function of the compression assembly discussed above, in particular by directing coolant or cooling effects to the cold-side heat exchanger of the TEG 20.
The key components of the TEG are the thermoelectric (TE) modules which convert the heat flux into electric power. The generalized configuration and operation of a typical TE module 50 is depicted in the diagram of FIG. 2. The thermoelectric module 50 includes alternating n-type and p-type semiconductor legs, 52 and 54, respectively, that are electrically connected 56 in series. The legs and electrical connections may be sandwiched between a cold side substrate 60 and a hot side substrate 61 within the TEG unit 20. The temperature gradient drives the electric current 58 in each leg according to the Seebeck effect, Eemf=−s ∇ T, where S is the Seebeck coefficient which is a property of the local material, and ∇ T is the temperature gradient across the semiconductor legs).
The efficiency of heat to electricity conversion is a material property characterized by dimensionless figure of merit ZT:
ZT=σS2T/(κel+κlat), where σ is the electrical conductivity, S is the Seebeck coefficient, T is temperature, κel is the electronic thermal conductivity, and κlat is the lattice thermal conductivity.